1. Field of the Invention
The present invention relates generally to a transmission/reception apparatus and method for improving throughput in a Multi-Input Multi-Output (MIMO) communication system.
2. Description of the Related Art
Next-generation communication systems have introduced MIMO to increase the capacity of wireless channels operated with limited frequency resources. The widely used Long Term Evolution (LTE) and Wireless Broadband (WiBro) standards are based on MIMO.
Additionally, the next-generation communication systems increase spectral efficiency or frequency efficiency by adaptively allocating a modulation order and an error-correcting code to channels between a transmitter and a receiver based on Adaptive Modulation and Coding (AMC). Meanwhile, to increase the system throughput by improving reception performance, the receiver takes advantage of Successive Interference Cancellation (SIC) that uses decoding results of one transport layer.
FIG. 1 shows a transmitter structure for large-delay Cyclic Delay Diversity (CDD) precoding in a MIMO communication system. The large-delay CDD is reflected in or applied to open loop spatial multiplexing, and this technology decentralizes a transport layer to all virtual antennas, contributing to a reduction in the amount of Channel Quality Information (CQI) or feedback information and the robustness of its accurate feedback.
Referring to FIG. 1, the transmitter includes a Modulation and Coding Scheme (MCS) selector 101, a first Cyclic Redundancy Check (CRC) error adder 103, a first Forward Error Correction (FFC) encoder 105, a first modulator 107, a resource allocator 109, a first Inverse Fast Fourier Transform (IFFT) unit 111, a Serial-to-Parallel (S/P) converter 113, a second CRC error adder 115, a third CRC error adder 117, a second FEC encoder 119, a third FEC encoder 121, a first rate matcher 123, a second rate matcher 125, a second modulator 127, a third modulator 129, a precoder 131, and a second IFFT unit 133.
Upon receipt of CQI feedback information 100 of each user, the MCS selector 101 determines an MCS level for each user's transport block based on the received CQI feedback information 100, and then outputs the MCS level to the first CRC error adder 103, an FEC encoding block 118, a rate-matching block 122, and a modulation block 126. As used herein, the transport block represents an independent information block undergoing encoding. It is assumed in FIG. 1 that there are two transport blocks and the transport blocks are equivalent to a codeword.
The first CRC error adder 103 generates a control signal to be transmitted to a receiver by integrating the MCS level of each user's transport block and each user's control information 110, adds to the control signal a CRC code for detecting an error occurring in a transmission process, and outputs the CRC-added control signal to the first FEC encoder 105.
The first FEC encoder 105 receives a signal output from the first CRC error adder 103, performs thereon FEC encoding for correcting an error occurring due to noise, using an FEC code, and outputs the FEC-encoded signal to the first modulator 107. There is no limit on the type of the FEC code. Generally, convolutional codes or trellis codes may be used as an FEC code for CQI feedback information.
The first modulator 107 receives a signal output from the first FEC encoder 105, maps the received signal to a signal constellation point, and outputs the mapped signal to the resource allocator 109.
Meanwhile, upon receipt of each user's traffic information 102, the S/P converter 113 divides the received traffic information 102 into N pieces of traffic information, where N is the number (i.e., two) of each user's transport blocks, and outputs the divided two pieces of traffic information to the second and third CRC error adders 115 and 117, respectively. The second and third CRC error adders 115 and 117 add to their input signals a CRC code for detecting an error occurring in the transmission process, and output the CRC-added signals to the second and third FEC encoders 119 and 121, respectively. The second and third FEC encoders 119 and 121 receive signals output from the second and third CRC error adders 115 and 117, respectively, perform thereon FEC encoding for correcting errors due to noise, using an FEC code, and output the FEC-encoded signals to the first and second rate matchers 123 and 125, respectively. As stated above, there is no limit on the type of the FEC code. Generally, convolutional codes, turbo codes, or Low Density Parity Check (LDPC) codes may be used as an FEC code for traffic information.
The first and second rate matchers 123 and 125 receive signals output from the second and third FEC encoders 119 and 121, respectively, perform thereon rate matching so that the number of bits of the input signals may be matched to the number of modulation symbols allocated to each user, and output the rate-matched signals to the second and third modulators 127 and 129, respectively.
The second and third modulators 127 and 129 receive signals output from the first and second rate matchers 123 and 125, respectively, map the received signals to a signal constellation point, and output the mapped signals to the precoder 131.
The precoder 131 receives each of the signals output from the second and third modulators 127 and 129, generates a traffic channel signal by precoding the received signals according to a predetermined rule, for example, large-delay CDD, and outputs the traffic channel signal to the resource allocator 109. A modulation order used in the second and third modulators 127 and 129 is determined depending on the MCS level selected by the MCS selector 101, and the CRC error adders, the FEC encoders and the modulators used for the CQI feedback information 100 and the traffic information 102 are generally different from one another.
The resource allocator 109 reorders a traffic channel signal, a control channel signal and a pilot signal, adjusts power levels of respective channels according to the power ratio capable of guaranteeing reception performance of the channels, applies the adjusted power levels to the reordered signals, and outputs the power-adjusted signals to the first and second IFFT units 111 and 133. The first and second IFFT units 111 and 133 transform frequency-domain signals output from the resource allocator 109 into time-domain signals, and output the time-domain signals via their transmit antennas.
FIG. 2 shows a structure of a receiver, which corresponds to the transmitter of FIG. 1 and to which SIC is applied (hereinafter referred to as an “SIC receiver”), in a MIMO communication system.
Referring to FIG. 2, the SIC receiver includes a first Fast Fourier Transform (FFT) unit 201, a second FFT unit 203, a resource deallocator 205, a layer ordering unit 207, an equivalent channel generator 209, a MIMO demodulator 211, a rate dematcher 213, an FEC decoder 215, a CRC error detector 217, a control channel detector 219, a CQI metric generator 221, a CQI generator 223, an FEC encoder 231, a rate matcher 233, and a modulator 235. The components other than the CQI metric generator 221 and the CQI generator 223 may be called a “signal receiver” since they are for signal reception.
The first and second FFT units 201 and 203 transform time-domain signals received via their receive antennas, into frequency-domain signals, and output the frequency-domain signals to the resource deallocator 205. The resource deallocator 205 separates signals output from the first and second FFT units 201 and 203 into a traffic channel signal and a control channel signal, and outputs the traffic channel signal to the MIMO demodulator 211 through the equivalent channel generator 209 and the control channel signal to the control channel detector 219.
The control channel detector 219 detects an MCS level for each user's transport block, resource allocation information of the traffic channel, the number of transport layers, necessity of retransmission, etc., which are needed for the reception of the traffic channel signal, using the control channel signal and channel estimate output from the resource deallocator 205, and delivers the detected information to the blocks requiring the information. In particular, the control channel detector 219 detects the MCS level for each user's transport block, and outputs the detected MCS level to the resource deallocator 205, the MIMO demodulator 211, the rate dematcher 213 and the FEC decoder 215.
The layer ordering unit 207 determines one of the two transport blocks carrying traffic channel signals, which is to be decoded first. Generally, the layer ordering unit 207 determines the channel status for each of transport blocks, using an equivalent channel value output from the equivalent channel generator 209, a Log Likelihood Ratio (LLR) value output from the MIMO demodulator 211, and error detection results output from the CRC error detector 217, and determines a decoding order so that transport blocks may be decoded in order of better channel status. The equivalent channel generator 209 reflects in the equivalent channel value the result caused by precoding the transmission signal by large-delay CDD in accordance with a predetermined rule, based on the number of transport layers, detected by the control channel detector 219, and the resource allocation information for the traffic channel.
For a transport block that is determined to be decoded first in the decoding order by the layer ordering unit 207, the MIMO demodulator 211 receives an equivalent channel value of the equivalent channel generator 209 and a received signal on the traffic channel, generates an LLR value using a MIMO receive algorithm, and outputs the generated LLR to the rate dematcher 213.
The rate dematcher 213 receives a signal output from the MIMO demodulator 211, and performs rate dematching on the received signal, and the FEC decoder 215 FEC-decodes the rate-dematched signal, and outputs the FEC-decoded signal to the CRC error detector 217. The CRC error detector 217 detects an error occurring in the transmission process from the signal decoded by the FEC decoder 215 using the CRC technique, and determines necessity of retransmission. If no error is detected, the CRC error detector 217 delivers the decoded signal to an upper layer. The MIMO receive algorithm may include a linear receive algorithm based on Minimum Mean Square Examination (MMSE) or QR decomposition, and an algorithm based on Maximum Likelihood (ML).
If no CRC error is detected in the first decoded transport block, an LLR value for a transport block that is determined to be decoded next, i.e., secondly, in the decoding order by the layer ordering unit 207, may be generated by a regeneration block 230. That is, if a transmission signal for the first decoded transport block is generated and delivered to the MIMO demodulator 211, then the MIMO demodulator 211 generates an LLR value for a second transport block by removing the transmission signal for the first decoded transport block from the received signal on the traffic channel, and outputs the generated LLR to the rate dematcher 213. The rate dematcher 213 receives a signal output from the MIMO demodulator 211 and performs rate dematching on the received signal, and the FEC decoder 215 FEC-decodes the rate-dematched signal and outputs the FEC-decoded signal to the CRC error detector 217. The CRC error detector 217 detects an error occurring in the transmission process from the signal decoded by the FEC decoder 215 using the CRC technique, and determines necessity of retransmission. If no error is detected, the CRC error detector 217 delivers the decoded signal to the upper layer.
It is assumed herein that no CRC error is detected in the first decoded transport block. In this way, if no CRC error is detected in the first decoded transport block, the transmission signal for the first decoded transport block is removed from the received signal on the traffic channel using the regeneration block 230, and this scheme is called “SIC scheme.” If a CRC error is detected in the first decoded transport block, an LLR value of a second decoded transport block is also generated in the same method as the first decoded transport block.
The CQI metric generator 221 generally estimates a CQI metric value such as an effective Signal to Interference plus Noise Ratio (SINR) using the equivalent channel value output from the equivalent channel generator 209, and outputs the estimated CQI metric value to the CQI generator 223. While it is assumed herein that the CQI metric value is generated using the equivalent channel value, the CQI metric value may be generated using the LLR value output from the MIMO demodulator 211.
The CQI generator 223 receives the CQI metric value output from the CQI metric generator 221, quantizes the CQI metric value, and generates a CQI index to be reported to a transmitter. In generating the CQI index, the CQI generator 223 may use ACKnowledgement (ACK)/Negative ACKnowledgement (NACK) information received from the CRC error detector 217. The generated CQI index is fed back to the transmitter along with feedback information.
Meanwhile, if signals output from the second and third modulators 127 and 129 in FIG. 1 are defined as [x1, x2]T, traffic channel signals [y1, y2]T being output through the precoder 131 can be represented by Equation (1) as follows:
                              (                                                                      y                  1                                                                                                      y                  2                                                              )                =                              (                                                            1                                                  0                                                                              0                                                                      e                                                                  -                        π                                            ⁢                                                                                          ⁢                      i                                                                                            )                    ⁢                      1                          2                                ⁢                      (                                                            1                                                  1                                                                              1                                                                      -                    1                                                                        )                    ⁢                      (                                                                                x                    1                                                                                                                    x                    2                                                                        )                                              (        1        )            where i represents indices 0, 1 and 2 of Resource Elements (REs) through which traffic channel signals are transmitted.
Additionally, equivalent channels that x1 and x2 actually undergo in the receiver of FIG. 2, can be defined as follows in Equation (2):
                                                                        (                                                                                                    h                                                  e                          ,                          11                                                                                                                                    h                                                  e                          ,                          21                                                                                                                                                                        h                                                  e                          ,                          12                                                                                                                                    h                                                  e                          ,                          22                                                                                                                    )                            =                                                (                                                                                                              h                          11                                                                                                                      h                          21                                                                                                                                                              h                          12                                                                                                                      h                          22                                                                                                      )                                ⁢                                  (                                                                                    1                                                                    0                                                                                                            0                                                                                              e                                                                                    -                              π                                                        ⁢                                                                                                                  ⁢                            i                                                                                                                                )                                ⁢                                  1                                      2                                                  ⁢                                  (                                                                                    1                                                                    1                                                                                                            1                                                                                              -                          1                                                                                                      )                                                                                                        =                                                1                                      2                                                  ⁢                                  (                                                                                                                                          h                            11                                                    +                                                                                                                    h                                21                                                            ⁡                                                              (                                                                  -                                  1                                                                )                                                                                      i                                                                                                                                                                            h                            11                                                    -                                                                                                                    h                                21                                                            ⁡                                                              (                                                                  -                                  1                                                                )                                                                                      i                                                                                                                                                                                                                    h                            12                                                    +                                                                                                                    h                                22                                                            ⁡                                                              (                                                                  -                                  1                                                                )                                                                                      i                                                                                                                                                                            h                            12                                                    -                                                                                                                    h                                22                                                            ⁡                                                              (                                                                  -                                  1                                                                )                                                                                      i                                                                                                                                )                                                                                        (        2        )            
As can be understood from Equation (2), assuming that adjacent REs are the same in actual channel characteristic, since x1 and x2 undergo equivalent channel characteristics, the CQI feedback information being transmitted to the transmitter to which large-delay CDD is applied, carries only one value rather than carrying a separate value for each transport block. Therefore, in determining an MCS level for each transport block, the transmitter also determines the same MCS level for different transport blocks.
However, if every transport block has the same MCS level in the transmitter to which large-delay CDD is applied, the SIC receiver may disadvantageously fail to achieve its potential capabilities to improve throughput.